Malignant gliomas are the most common type of primary malignant brain tumors. They are characterized by enhanced growing capabilities, neoangiogenic proliferation, and extensive infiltration of the brain parenchyma, which make their complete surgical resection impossible. Together with transient and refractory responses to standard therapy, these aggressive neoplasms are incurable and present a median survival of 12 to 14 months. Transforming growth factor-beta (TGF-β) is a pleiotropic cytokine of which two of the three isoforms expressed in humans have been shown to be overexpressed proportionally to the histologic grade of glioma malignancy. The increase of chromosomal aberrations and genetic mutations observed in glioma cells turns TGF-β into an oncogene. For that reason, it plays critical roles in glioma progression through induction of several genes implicated in many carcinogenic processes such as proliferation, angiogenesis, and invasion. Consequently, investigators have begun developing innovative therapeutics targeting this growth factor or its signaling pathway in an attempt to hinder TGF-β's appalling effects in order to refine the treatment of malignant gliomas and improve their prognosis. In this paper, we extensively review the TGF-β-induced oncogenic pathways and discuss the diverse new molecules targeting this growth factor.

Glioblastoma (GBM) represents the most common and aggressive malignant primary brain tumors in adults. Response to standard treatment is transitory and the survival of clinical trial cohorts are little more than 14 months. GBM are characterized by excessive proliferation, invasiveness, and radio-/chemoresistance features; which are strongly upregulated by transforming growth factor-beta (TGF-β). We hypothesized that TGF-β gene expression could correlate with overall survival (OS) and serve as a prognostic biomarker. TGF-β1 and -β2 expression were analyzed by qPCR in 159 GBM tumor specimens. Kaplan–Meier and multivariate analyses were used to correlate expression with OS and progression-free survival (PFS). In GBM, TGF-β1 and -β2 levels were 33- and 11-fold higher respectively than in non-tumoral samples. Kaplan–Meier and multivariate analyses revealed that high to moderate expressions of TGF-β1 significantly conferred a strikingly poorer OS and PFS in newly diagnosed patients. Interestingly, at relapse, neither isoforms had meaningful impact on clinical evolution. We demonstrate that TGF-β1 is the dominant isoform in newly diagnosed GBM rather than the previously acknowledged TGF-β2. We believe our study is the first to unveil a significant relationship between TGF-β1 expression and OS or PFS in newly diagnosed GBM. TGF-β1 could serve as a prognostic biomarker or target affecting treatment planning and patient follow-up.

Thyroid hormone receptors (TRs) are transcription factor members of the nuclear receptor superfamily. The transcriptional activity of TRs is controlled by thyroid hormones and cell-specific coregulators. Using the yeast two-hybrid system, we identified RanBPM as a new protein partner for TRs. RanBPM was initially discovered as an interacting partner for Ran, and was also shown to be a protein partner and coactivator of the androgen receptor. The novel interaction between RanBPM and TR isoforms was addressed by glutathione-S-transferase (GST) pull-down assays and co-immunoprecipitation in intact mammalian cells, where RanBPM was shown to bind TRs in a ligand-independent fashion. We also studied the regions implicated in the interaction with deletion mutants: the principal interacting region of RanBPM is comprised within its carboxyl-terminal end and the TR DNA-binding domain is sufficient to mediate the interaction. To investigate the potential role of RanBPM in thyroid hormone action, transient transfections with luciferase reporter genes were performed in CV-1 cells. We found that the over-expression of RanBPM increases the activation of TRETK-and DR+4-positive thyroid hormone response elements. Interestingly, over-expression of the truncated protein RanBPM55, which lacks the N-terminal polyglutaminated region but binds TRs, decreased the fold activation by almost 80%. Furthermore, we performed competition assays using transient transfection of RanBPM and increasing amounts of RanBPM55. This revealed that the stimulating effect on TR transactivation by the full-length protein is inhibited in a dose-dependent fashion by RanBPM55. This suggests that although the polyglutaminated region of RanBPM is not required for the binding to TRs, it is required for the stimulation of TR transactivation. Taken together, our results provide evidence that RanBPM is a potent novel coactivator for thyroid hormone receptors.

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